It is not known precisely when the Celts first enter Britain in their steady expansion outwards from central Europe. But Caesar states, in his own account of his campaigns, that they have been migrating across the Channel since at least the 2nd century BC.

Caesar makes his first tentative excursion to Britain in August of 55 BC. He lands on the coast of Kent, meeting considerable opposition from the cavalry and war chariots of the neighbouring Celtic chieftains. After staying long enough to demonstrate to the British the strength of a Roman legion, he returns in September to Gaul.

During the winter Caesar builds 600 new ships. He sails again, in July of 54 BC, with five legions and 2000 cavalry. They are sufficient to bring him north of the Thames into the territory of Cassivellaunus, the tribal chieftain chosen to lead the British forces. Caesar easily captures the Celtic leader's primitive stronghold, and removes from it a large herd of cattle. But by the time he sails away again, in September, little has been achieved - except that Cassivellaunus has agreed to a treaty and has promised an annual tribute. It is unlikely that any tribute is paid.

The Celtic chieftains of Britain have almost exactly a century before they are again disturbed by the Romans.

Celtic Britain: 1st century BC - 1st century AD

The Celtic kings of southern Britain make good use of the years following Caesar's incursions. His failure to do more than come and see, without conquering, convinces them that the Channel is a safe defence. The natural extremity of the Roman empire is the coast of Gaul.

Even Gaul is hard for the Romans to hold. After Caesar's conquest of Gaul there are several uprisings by local chieftains. They are encouraged in this by the Celtic chieftains of Britain, their kinsmen and - against Rome at least - their natural allies. Yet increasing contact with Roman civilization is at the same time bringing wealth and sophistication to Britain.

The Celts of Britain benefit, through trade, from the proximity of Roman Gaul. It is a familiar pattern of international commerce that raw materials move inwards from the primitive extremities of a region, in return for manufactured goods sent back from the centre.

Across the Channel from Britain go gold, silver, iron, grain, wool, hides and cattle (a list to which a contemporary author, Strabo, adds hunting dogs and slaves). Back from Rome come glass, jewellery and other luxuries.

The rulers of the Celtic tribes of Britain become, during this period, more prosperous and more powerful - but, in most cases, no more friendly to Rome. The tendency is personified in a chieftain regarded by the Romans, during the reign of Augustus, as the king of Britain.

The Romans call him Cunobelinus; his Celtic name is Cunobelin; he is famous in English as Cymbeline. He may be either the grandson or great grandson of Cassivellaunus, who faced Caesar's invasion. In a reign of about thirty-five years Cymbeline cunningly avoids provoking the Romans, while offering them no concessions. The man is clearly dangerous. The conquest of his large offshore island is increasingly seen in Rome as a necessary task.

The event which finally precipitates the invasion is the death of Cymbeline soon after AD 40. One of his sons, Amminius, known to be pro-Roman, has recently been exiled by the stronger anti-Roman faction at Cymbeline's court. Amminius goes to Rome for help, during the reign of the emperor Caligula. When Cymbeline dies, two other sons - known to be anti-Roman - inherit his power. They are Caractacus and Togodumnus.

By the time the Romans are ready to invade, in AD 43, Claudius has recently been chosen as emperor. With a reputation for feebleness, he needs a striking success of some kind. He takes a personal interest in the campaign against Britain.

The Roman conquest of Britain: AD 43-51

Four Roman legions land in Kent in AD 43. The two sons of Cymbeline attempt to hold them at the Medway but are defeated (an engagement in which Togodumnus is killed, leaving Caractacus in sole command of the British forces). The Britons then retreat beyond the Thames, at which point the Romans call a halt in their pursuit. They are waiting for the public-relations part of the exercise.

A few weeks later the emperor Claudius reaches the southern bank of the Thames, in the region of what is now London, with fresh troops and even a few elephants. He is here to lead the advance on Caractacus' capital at Camulodunum, or Colchester.

There is little further opposition, for the Celtic troops - without breastplates or helmets - are no match for the solid weight of a Roman legion, advancing like a human tank. The emperor enters Colchester in triumph, cheered by his army. Later a temple is erected here to Claudius as a god; its site is now Colchester Castle.

After the rapid defeat of Caractacus, chieftain of the Belgae in southeast Britain, other Celtic tribes quickly come to terms with the Romans. Some are accepting defeat. But others, such as the Iceni in East Anglia, already have friendly relations with the Romans - preferring them to the Belgae. Rome leaves such chieftains in power, as allies.

The result is that in the short space of four years the whole of southern Britain is safely under Roman control. In AD 47 Roman troops are able to build a raised road, with a ditch on either side, defining the northern edge of this safe territory. Known as the Fosse Way, it stretches from Lincoln to south Devon.

But beyond the Fosse Way there is trouble for the Romans in the shape, once again, of Caractacus. He has escaped alive from his defeats. Now he is organizing resistance among the Welsh tribes. Caractacus himself is captured in AD 51, but the Romans are unable to subdue the Welsh for another thirty years.

Like all barbarian kings captured by the Romans in war, Caractacus - accompanied by his wife and daughter - is taken back to Rome. The family is to be displayed before the Roman crowd in the triumphal celebration of the conquest of Britain.

Normally, as with Vercingetorix a century earlier, the captive king would then be executed. But the Noble bearing of Caractacus, and his powerful speech to the assembled crowd, so impress the emperor that Claudius spares his life and that of his family. He provides them with a villa in Rome, where they live a guarded but honourable existence.

Boudicca and the Iceni: AD 60-61

The only major threat to Roman dominance of southern Britain derives from their own heavy-handedness.

The Iceni, a tribe of Celts occupying what is now Norfolk, have been allies of the Romans. Their king, Prasutagus, has no male heir. In an attempt to ensure a good relationship between his family and the Romans, he leaves a will dividing his wealth between his two daughters and the emperor Nero. It does not have the desired effect. On his death, in AD 60, his kingdom is annexed by the Romans; his family is humiliated; and the lands of the tribe are plundered. But the Romans have not taken account of his widow. In Latin they spell her Boadicea. Her Celtic name is Boudicca.

Boudicca launches an uprising in which she is soon joined by other Celtic tribes. All have good cause for resentment at the behaviour of Roman soldiers and Roman settlers in their territories. Together they attack Colchester, destroying the Roman garrison which attempts to defend itself in the newly completed temple to Claudius. They plunder many other rich settlements before moving on to ravage Verulamium (now St Albans) and London. According to Tacitus, 70,000 are killed.

Eventually the Romans gather together an army of about 10,000 men to confront the tribes - now busy in another cause, the quest for loot.

Tacitus paints a touching picture of the Celtic tribesmen milling about in confusion as they face the solid Roman formation on the battlefield. Their families have arrived in carts to watch the encounter. Boudicca dashes among her people in a chariot, accompanied by her two daughters - whose rape by Roman soldiers, according to Tacitus, has sparked off the crisis. The harangue to the troops which Tacitus puts into the mouth of the queen begins: 'We British are used to Woman commanders in war'.

But she cannot prevail. Tacitus claims that after a crushing defeat she takes poison. Hers is the last serious uprising in southern Britain. The attention of the Romans can be turned to Wales.

The campaigns of Agricola: AD 77-84

Little progress is made in pacifying Wales until the arrival in Britain of Agricola. More is known of Agricola than of any other Roman general of comparable stature, because he takes the wise precaution of having a historian as a son-in-law. Agricola's appointment as governor of Britain and the marriage of his daughter to Tacitus occur in the same year - AD 77.

Agricola rapidly succeeds in conquering the Welsh tribes, even in Anglesey. To consolidate his gains he stations the 20th legion in an encampment on the river Dee. Castra Devana ('camp on the Dee') becomes one of the most important Roman strongholds in Britain. Its modern name, deriving from 'Castra', is Chester.

In AD 78-9 Agricola brings the north of England under Roman control. In 80 he establishes a line of defensive outposts across Scotland's narrowest point, between the Clyde and the Forth. In the following three years he presses steadily further north into the wilds of Caledonia (the Roman word for Scotland, from the name of its leading tribe). Finally, in AD 83, he wins a major victory over the Caledonii at an unidentified place called Mons Graupius - probably almost as far north as Aberdeen.

Meanwhile Agricola has also very effectively governed the rest of Britain. It has been an impressive seven years. It is lucky indeed that there is a historian in the family to record them.

Tacitus explains that his father-in-law has to deal in Britain with people 'living in isolation and ignorance' who are therefore 'prone to fight'. As a distraction Agricola introduces the Celts to the trappings of Roman luxury. Yet baths and sumptuous banquets, the historian candidly admits, are merely another aspect of Britain's enslavement.

In the same vein, the son-in-law reveals that Agricola dreams of conquering Ireland. He believes that it could be controlled by a single legion, and that it would be 'easier to hold Britain if it were completely surrounded by Roman armies, so that liberty was banished from its sight'. It never happens. Ireland (or Hibernia), alone in western Europe, remains free of the Romans.

Emperors building walls: AD 122-142

Water has until now provided the natural boundaries of the Roman empire in Europe - the Atlantic, the Rhine and the Danube. With the invasion of Britain, followed by the failure to conquer the whole island, another form of frontier against northern barbarians becomes essential.

It is provided by the emperor Hadrian, who visits Britain in 122. Deciding that the advances made by Agricola far into Caledonia are untenable, he orders the construction of a defensive barrier stretching seventy-five miles from coast to coast across the north of what is now England. Hadrian's Wall remains even today a massively impressive structure. It takes the Romans only about eight years to complete it.

In subsequent years the Romans again push north of the wall, encouraging Hadrian's successor Antoninus Pius to order the construction of another barrier further into Caledonia. The Antonine Wall, built from about 142, is an earthwork on stone foundations across the narrowest part of Britain - the forty miles between the Clyde and the Forth.

This further line proves impossible to hold, so Hadrian's Wall becomes the northern frontier of Roman civilization. Its existence, and the Roman presence south of it, has a profound influence on the histories of England and Scotland - though the border between them is eventually established a little to the north.

Britannia: 2nd - 4th century AD

Hadrian's Wall, established from the 2nd century AD as the frontier of Roman rule in the British Isles, enables England and Wales (as they will later become) to settle down together as Britannia, the most northerly Roman province.

On the whole the Celtic chieftains of Britain adapt willingly to Roman customs and comforts. They learn to live in villas, they speak Latin, they benefit from trading links with the empire (British wheat and wool are much in demand), and they become Roman citizens. The tribal centres develop into thriving Roman towns, around the forum (market place) and basilica (town hall).

Towns of this kind, serving as the capitals of British tribal rulers enjoying Roman support, include Winchester, Dorchester, Cirencester and Canterbury. London develops at the same period, but as a centre of trade at the focal point of the network of Roman roads. Bath, with its hot springs, becomes Britain's first resort.

Different in kind are the essentially Roman headquarters of Chester, Caerleon and York (where Constantine is proclaimed emperor in 306). These are the permanent bases of the Roman legions in Britain. Other modern cities, including Lincoln, Colchester and St Albans, derive from Roman municipalities - founded for new settlers, such as men retiring from the legions.

Roman Britain never achieves the prosperity or sophistication of Roman Gaul, and it has the disadvantage of being cut off from the centre whenever Gaul is controlled by rebellious Roman armies or invading barbarians. Even so, Britain has much in common with other provinces of the empire. It has its great villas (a palace at Fishbourne, discovered in 1960, is one of the grandest, with superb mosaic floors). And it has its choice of the empire's rival religions.

By the late 3rd century Mithras and Jesus Christ compete for attention. In 314 the winning side, the Christians, are sufficiently well organized to send three bishops from Britain to a council in Gaul.

Britannia in decline: 5th - 6th century AD

The decline of Roman Britain is like the withering of a limb at the extremity of an ailing body. In unsettled times, in the late 4th century, western emperors withdraw legions from Britain for their own local purposes. Once Gaul is in the hands of barbarian rulers in the 5th century, blocking the route from Rome, no new replacements arrive.

The Roman British find themselves extremely vulnerable. They have defences in the north, but none in the southeast - the direction of Rome, and supposedly secure. It is from this undefended side that danger comes. German tribes moving south and west into Gaul have Britain in their sights.

The main threat is from two tribal groups pressing southwest from the Baltic coast. They are the Angles and the Saxons. The subsequent Anglo-Saxon basis of England, and of the English language, speaks for their success.

The Romanized Celts, deprived of their Roman legions, prove unable to resist these more primitive and ferocious intruders - though their struggle is personified in a legendary hero, King Arthur. By the 6th century the Celtic chieftains are confined to mountainous Wales. The fertile plains of England are occupied now by Angles, Saxons and other German tribes from roughly the same area, such as Jutes and Frisians. Their chieftains set about establishing themselves as regional kings.